1. Field of the Invention
This invention relates to sputtering targets useful, for example, for depositing thin films by DC magnetron sputtering. Such thin films, in turn, can be used in the manufacture of optical recording media, cutting tools, eyeglass lenses, read/write heads for magnetic recording, photovoltaic cells, semiconductors, and automotive and structural glass.
2. Description of the Related Art
Various techniques for physical vapor deposition of thin films are known, including evaporation and various sputtering processes. Commonly employed sputtering processes include diode sputtering and magnetically enhanced sputtering.
In typical sputtering processes an electric field is maintained between an anode and a target cathode within a partial vacuum (i.e., a pressure from about 1.times.10.sup.-4 to 3.times.10.sup.-2 torr). A gas such as argon is ionized between the anode and the cathode, forming a plasma. Free electrons in the plasma are accelerated toward the anode, colliding with other gas atoms in their path to further ionize the gas. The gas ions are accelerated to the target cathode and dislodge or sputter atoms by transferring their momentum to the atoms near the surface of the target. The substrate to be coated is placed in proximity to the cathode, and the sputtered atoms traverse the space between the cathode and substrate and condense on the substrate.
In magnetically enhanced sputtering a magnetic field is applied perpendicular to the electric field near the cathode surface. The magnetic field causes electrons in the plasma to follow curved trajectories. A longer (curved) path increases the probability of collision with gas atoms in the chamber. Such collisions produce additional gas ions thereby increasing the sputtering rate.
Another feature of the magnetic enhancement is that electrons are confined near the cathode surface, thereby increasing plasma density in this region. This increase in plasma density enables operation at lower gas pressures and decreases electron bombardment heating of the substrate. The latter feature is particularly important when substrate materials having relatively low melting points are used.
In one type of magnetically enhanced sputtering, known as magnetron sputtering, the electron confinement field is such that a closed torroidal electron trapping path region or "racetrack" is formed adjacent to the sputtering target surface. Magnetron sputtering typically exhibits good sputtering efficiency and is commonly used for thin film deposition.
Magnetron sputter-deposition processes typically operate at discharge voltages of greater than 100 but less than 1000 volts. During normal operation, the discharge current is distributed over a region defined by the magnetic confinement field geometry. One mode of undesirable operation during magnetron sputter deposition is known as arcing. When arcing occurs, the discharge voltage generally decreases and the discharge current becomes localized to a small region of the target surface area.
For the deposition of optical thin films, the occurrence of arcing during sputter deposition processes is typically undesirable. The intense, localized discharge associated with arcing can cause high local deposition rates, local melting of the target surface, spattering of molten material, and cracking or disintegration of the target. Further, in general, frequent arcing during deposition causes numerous defects in the deposited films and substantially reduces target lifetimes.
Magnetron sputtering can be characterized by the power source employed. "DC magnetron sputtering," which utilizes a direct current power source, requires a target having sufficient electrical conductivity to transport the discharge current with an acceptably low voltage drop. Assuming a target current density of 50 mA/cm.sup.2, and bearing in mind that an induced through-target voltage drop greater than 50 volts is unacceptable, a maximum target material resistance of 1000 ohm/cm.sup.2 is required. For a target thickness of 0.3 cm, the permissible resistivity is approximately 3300 ohm-cm or less.
Another type of magnetron sputtering, known as "RF magnetron sputtering," has also been used to provide sputtered films. RF magnetron sputtering, which uses a radio frequency power source, does not require an electrically conductive sputtering target such as that needed for DC magnetron sputtering.
A variation of magnetron sputtering, called reactive sputtering, has been used to deposit films for which use of a bulk target with the desired film composition has, for example, low electrical or thermal conductivity, and poor mechanical properties, or exhibits low deposition rates. In reactive sputtering, a target containing some of the film constituents is sputtered in an atmosphere containing one or more additional film constituents. Reaction of the sputtered target constituents with the gaseous constituents to form the final (typically, electrically insulating) film composition can then occur either on the substrate surface or in the gas phase. An example of a reactive deposition process is the formation of alumina thin films by sputtering an aluminum target using an Ar-O.sub.2 working gas mixture.